Induction coupled plasma (ICP) generators typically are used for spectroscopy, treatment of fine powders, melting of materials, chemical reactions and the like. These applications derive from the high temperatures inherently associated with a plasma, which are high enough to effect electron excitation and ionization of the plasma gas and injected materials. Plasma generation also is utilized to produce ions utilized in instruments for mass spectroscopy and, at low pressure, for vapor deposition processes. ICP generators are driven by high frequency electronic oscillators by coupling through a load coil, broadly in the radio frequency range of 10 to 100 MHz. A problem associated with the high frequency is radiation that can interfere with auxiliary equipment such as radios, computers and videos and related control equipment.
ICP generators generally are enclosed in a housing compartment that serves several purposes including isolating the plasma from atmospheric air, protecting personnel from light radiation and containing radio frequency radiation. The generator includes tubes for conveying a plasma forming gas through the coil, and for introducing sample material into the plasma. Such sample is derived from a source such a nebulizer that generally is mounted outside of the housing compartment for protection against the heat of the plasma. There is a need to service and substitute components of the plasma generator including the tubes and the nebulizer or other sample source. In existing apparatus the housing must be opened for procedures inside such as removal and replacement of components, which is cumbersome because the gas and sample tubes extend into the coil.
A problem may occur if the tip of the sample injection tube is not located precisely, particularly in the case of an organic solvent being used to carry dissolved sample material. Heat from the plasma discharge can cause a carbon deposit to form on the tip if the tip is to close to the base of the plasma. This deposit interferes with analytical performance by causing the injector to block the flow of sample to the plasma. On the other hand, if the tip is too far from the plasma, the sample disburses too much and goes around the plasma instead of into it. It is difficult to find a proper position without significant trial and error, requiring disassembly each time. Thus, there is a need for adjustment of the axial position of gas tubes with respect to a plasma-forming load coil during operation, in order to optimize the tip location.
An object of the invention is to provide a novel mounting apparatus allowing easy removal and replacement of components of an induction coupled plasma generator. Another object is to provide a mounting assembly of such components which is easily removed from and replaced in a housing assembly. A further object is to provide such removal and replacement without tools. Yet another object to provide such a mounting apparatus with shielding of radio frequency emission. Another object is to provide such a mounting apparatus that allows adjustment of axial position of gas tubes with respect to a plasma-forming load coil, during plasma generation. Another object is to provide a mounting apparatus that allows for automatic contacting of a spark or high frequency starting lead during the replacement.